The first enzymatic step common to the biosynthesis in plants of the branched chain amino acids (valine, leucine and isoleucine) is catalyzed by the enzyme acetohydroxyacid synthase (AHAS; also known as acetolactate synthase; E.C.4.1.3.18). AHAS catalyzes two parallel reactions: condensation of two moles of pyruvate to give rise to acetolactate, and condensation of a mole of pyruvate and a mole of alpha ketobutyrate to yield acetohydroxybutyrate. This enzyme is inhibited by the end products of the pathway (valine, leucine and isoleucine) and this is one of the known mechanisms of regulation of this pathway in higher plants.
AHAS is the target site of several classes of structurally unrelated herbicides. These herbicides include the imidazolinones, the sulfamoylureas, the sulfonylcarboxamides, the sulfonamides and the sulfonylureas.
Large scale commercial agriculture relies heavily on row-crop production practices. The availability of herbicides which selectively eliminate problem weeds while leaving crop plants undamaged is a major enabling component of these practices. Herbicides which control the majority of problem weeds are available for most major crops. The afore-mentioned AHAS inhibiting herbicides are a key element in weed control. These same herbicides, however, may miss important weeds in certain niche crop production areas. Also, currently used herbicides may have ecological problems or cost constraints attached to their use.